Can you use a normal deck of cards in the routine? Force any of the "4"s using your favorite card force and then tell them to now think of the corresponding letter from the alphabet (Ace = A, 2 = B, 3 = C, 4 = D .... )

How about a change Bag? One side full of folded up pieces of paper with all the different letters (except "D"), the other side with all "D"s. Let them reach in and pick one.

Not sure if it qualifies as "clean" but the "Elephants in Denmark" stunt that went viral a few years ago gets there as follows:

1: ask each person to think of a number from 1 to 10.
2: take that number and multiply by 9
3: take the result and add the number together (i.e. 72 = 7+2, 9 = 0+9).
4: take that number and subtract 5
5: take that result and equate it to the corresponding letter of the alphabet (i.e., 4 = D).

You could use a clear plastic change bag (easily found with Google). For the letters you could use Scrabble tiles, or even those beads with letters used to make baby-name bead bracelets (available at your local crafts store).